This invention relates to a method of product extrusion where the extruded product material is formed into discrete products which stabilize after extrusion of the product material.
This applies in the production of xe2x80x9csolidxe2x80x9d unexpanded products where it is desired to avoid expansion, and especially in the production of expanded products where the dimensions of the product are greater than the nozzle aperture(s), where it is aimed to achieve a specific dimension increase in relation to the nozzle dimension.
The problem presents itself mostly in the manufacture of expanded products where the expansion is normally aimed at in order to ensure specific characteristics, such as viscosity (fish-foods), sinking characteristics (specifically slow sinking speed of fish-foods), liquid absorption characteristics (fish-foods, pet-food, foodstuffs), constant low specific density (pet-food, snacks, breakfast products), constant crispness (fish-foods, pet-food; foodstuffs), constant physical shape (all products).
The ensuring of the above-mentioned constancy/control-lability of the expansion gives rise to great problems with all extrusion systems. The degree of expansion of the extruded product in the extrusion plant depends on conditions such as:
Composition of raw materials.
Characteristic/quality of raw materials.
Extrusion process parameters:
Dosing of the individual ingredients:
Flour mixing.
Liquids.
Steam.
Cooling/heating of worms/linings.
Speed of rotation of worm conveyor.
Configuration of the extruder""s worm conveyor.
Configuration of the extruder nozzle.
Wear state of the process parts.
Especially the variations in the raw material and the wear state of the extruder""s worm conveyor and linings give rise to variations in production which are difficult to predict.
Depending on the conditions which give rise to the product variations, it can be particularly difficult to ensure (via automatic process control and regulation, or manually), that the finished products correspond to the product specifications with reasonable tolerances.
During the extrusion process, products such as those described above containing starchy raw materials will, as a consequence of a process temperature of 100-200xc2x0 C., a moistness of 10-35% H2O, and a pressure of 2-80 bar in the extruder worm, typically gelatinize and convert to a plastic/elastic liquid-like form.
In its passage through the extruder""s nozzle arrangement, the product will move from process pressure (2-80 bar) to atmospheric pressure, at which the cutting-off of the product in the desired length normally takes place. The fall in pressure across the nozzle plate and the energy content of the product will result in a momentary evaporation of a part of the product""s water content in the opening of the extruder nozzles. The evaporation of the water forms steam pockets in the plastic/elastic mass, whereby the product expands. The fall in temperature brought about by the momentary evaporation results in a hardening of the product into a permanently firm and physical shape.
The expansion of the product normally stops,
when the steam pressure in the steam pockets achieves a balance in relation to the elasticity of the mass,
and/or further development of steam stops as a consequence of the consumption of energy involved in the evaporation resulting in a drop in temperature to a temperature corresponding to the boiling point of the mass,
and/or when all of the walls in the steam pockets burst as a consequence of the internal steam pressure, and the pressure in the pockets is hereby normalised, whereby the driving force for further expansion disappears.
It is characteristic for all known extrusion systems for the production of feeding stuffs, foodstuffs or the like that the nozzle and cutting-off system of the extruding process operate under constant pressurexe2x80x94normally atmospheric pressure or possibly slight under-pressurexe2x80x94created by associated aspiration systems or pneumatic transport systems.
To overcome or minimize the expansion control problems specified above, it has been found by the present invention that the extrusion with advantage can be effected into a pressure chamber at the outlet side of the nozzle plate, in which the product is shaped, cut-off and stabilised to a permanent form (expansion degree) before being discharged to atmospheric pressure. By regulation of the working pressure of the pressure chamber, it is possible to regulate the boiling point for the mass in the mouth of the nozzle arrangement, whereby the development of steam brought about by the extrusion can be controlled. This provides a corresponding possibility of controlling the driving force which brings about the expansion of the product.
In accordance with the invention, the new method will be able to be used for optimisation of the expansion degree and capillary structure in products from all known types of extruders (single-worm, double-worm, dry extruders, cooking extruders with/without preconditioning).
The invention will make it possible to use raw materials in the extrusion process which have hitherto been avoided because of control problems related with the raw materials.
This situation results in smaller needs for (dependence on) scanty resources/expensive ingredients, and the possibility to a greater extent of composing receipts (raw material compositions) on the basis of nutritional/cost evaluation, with less regard being paid to the performance of the receipt/raw materials from the point of view of extrusion.
In accordance with the invention, the method will make it possible to increase the capacity of extruders which are mounted with such equipment, since the practical exploitable capacity of many plants is today restricted by the limit of controllability of the product""s physical shape.